“You will see how the river changes once we go outside the national park,” says Mahesh Thawani of Mumbai March, a voluntary organisation working on issues in the Sanjay Gandhi National Park (SNGP). I am meeting Mahesh, a long-time resident of the area, to walk along the Dahisar River, which originates in the park at Tulsi Lake. As we make our way through enthusiastic throngs of morning walkers, the river is omnipresent—providing a serene setting to a group of senior citizens singing, a refuge for birds, holding a mirror to the green of the trees along its banks and the blue of the sky above.

As it flows outside the park, it first disappears beneath a road, only to emerge on the other side less than one-fourth of its width. The trees are replaced with cowsheds, buildings and further down, informal settlements and the railway line. Here, the river transforms into a gutter with grey-green sludge.

A repository of pollution

Of the four rivers in Mumbai—Mithi, Dahisar, Poisar, and Oshiwara—the Dahisar is the second longest, with a catchment area of 3,488 hectares. It covers 12 kilometres before joining the Arabian Sea via Manori Creek. Mumbai does not pay as much attention to it as the Mithi, which captured the city’s collective attention after the July 2005 floods. Yet, the Dahisar is equally a repository of sewage and industrial pollution.

“Look at the beginning. The water is so clear and pure,” says biologist Anjana Pant who was approached by Mumbai March to come up with a proposal for its rejuvenation. She explains that they chose the Dahisar as it is small but has all the problems of a big river like Mithi. She found that households along the river dump solid waste into it. Sewage from informal settlements finds its way into the river as they lack a sewage system. To add to this, the water quality was further deteriorating due to a dhobi ghat and cow sheds located along the river bank.

Read more: Walking along the 17-km long Mithi river: A look at the riverine ecosystem

Dayanand Stalin of the NGO Vanashakti reiterates this. According to him, of the total pollutants in the river, 70% is sewage, 20% is industrial waste, and about 10% is animal waste. A study which assessed the water quality at four places on the river found that the water has heavy metals such as chromium and lead. Lead concentration was 2.07 mg/L and 0.14 mg/L at  Borivali Station and Dahisar Bridge, respectively, both higher than the permissible limit of 0.1 suggested by the Central Pollution Control Board (CPCB). The study also showed high values of total suspended solids at these locations. Besides this, the water had high levels of chlorides, well above the permissible limits, posing a threat to aquatic life.

Researchers consider several parameters when assessing water quality, with one of the key factors being Biological Oxygen Demand (BOD). BOD refers to the amount of dissolved oxygen in water consumed by microorganisms as they decompose organic matter. Anjana’s study also found elevated BOD levels in the water, suggesting the presence of significant amounts of organic waste.

Multi-pronged approach needed

Experts recommend various localised solutions to address river pollution. Anjana, who has worked on other river rejuvenation projects, believes site-specific approaches will offer diverse solutions. One such solution, supported by both Stalin and Anjana, is bioremediation. This involves introducing a mix of bacteria into the river to break down pollutants, using organisms mostly native to the ecosystem. Anjana explains, “Like, you have sulfide-reducing bacteria, which would treat the sulfide. And then you have phosphate-reducing bacteria, and you can get rid of phosphate in the chemical form.”

Pollution is not the only issue. The course of the river has been manipulated over the years, and encroachments have reduced its width. The BMC has built retaining walls along the river banks to prevent flooding. “We have seen how concretisation has disconnected the river from the city. People are not able to access the river,” points out architect Vikram Pawar, who used to walk along the Dahisar River with his students.

Read more: As monsoon intensifies, fear grips citizens living on the banks of Poisar river

Access to the river

In a city with few public spaces, neglecting river edges during non-monsoon months is a missed opportunity. However, flood mitigation plans cannot be reactive, like after the July 2005 floods, nor should they be isolated solutions. They must be integrated into broader city development plans.

Vikram notes that despite plans for buffer zones along the river, people encroach on them, or the BMC builds parks under the guise of beautification. “If the park is at a higher ground, wherever the parks are just next to the river, they are disconnected from the river,” he says. He suggests designing buffer zones as part of the city’s landscape to create a green corridor. The corporation can implement safeguards during the monsoon, but keep them accessible to the public at other times.

Flooding issues

The dhobi ghat does not contribute to the river’s pollution, as its water is not used to wash clothes anymore, Mahesh adds.

Kallu, a washerman who washed clothes on the riverbank earlier, says, “It’s been more than four years since we stopped using the river water. Now they have built a wall and we can’t access it.” He believes that this was a good move on the part of the BMC as washing clothes there polluted the river. Moreover, not having access to the river has paved the way for them to switch to washing machines, making their work less labour-intensive.

He adds that despite the wall, flooding occurs during a heavy downpour. This happened in 2019, and their work area and the road in front of it were filled with water. When it rains this heavily, the informal settlements along the river in Daulat Nagar get inundated.

Although the dhobi ghat is not adding pollutants to the river, other problems persist.

Rejuvenation efforts

The BMC has undertaken the rejuvenation of rivers based on the recommendations of the Madhav Chitale Committee. The initiative also falls under BMC’s Brihanmumbai Storm Water Disposal (Brimstowad) project, which was started in the aftermath of the 2005 floods. The BMC has allotted ₹246 crores for the rejuvenation of the Dahisar River. This includes sewage treatment plants (STP) and the removal of encroachments along the riverbank. The corporation is building two STPs across the river.

The treated water will be partially returned to the river. Anjana argues that STPs should be part of a centralised system, located away from the river, with pipelines directing water to it. Rather than returning treated water to the river, it should be purified to a level suitable for reuse. She highlights that sewage contains minerals like phosphates, nitrates, and carbon, which are beneficial for agriculture. “When there is no electricity, untreated sewage bypasses treatment. Secondly, STPs can stop working or require repairs. What happens during that downtime?”

The BMC also claims that the rejuvenation project is taking time as it has to find suitable accommodation for the people living in the informal settlements along the riverbank.

“Our response has been quite outdated anyway,” believes Vikram. He says that the flood solutions being implemented are based on conditions prevalent in the 1980s, when rainfall was distributed over three months. But today, we see intense and short spells of rain. Besides, increased concretisation means more surface runoff, with water eventually going back into the river. Rather than a flood-resistant approach, we need a flood-resilient approach, he adds.

Also read:

• Despite filtration exercise, clean Mithi River a distant possibility
• Saving Bengaluru’s lakes: How citizen audits protect ecosystem assets
• Chennai’s last lung space: Nanmangalam Lake faces an ecological emergency